Hydrogen induced crack propagation in bainitic X70 steel in hydrogen pressured tensile test

Abstract

This study investigated hydrogen induced cracking (HIC) of X70 steel, used for gas pipeline material, with a bainitic microstructure. The effect of hydrogen on the steel was investigated by tensile-testing in a high pressure hydrogen atmosphere, using OM, SEM and TEM analyses. The mechanisms of hydrogen-related cracking were represented as stress-induced hydride formation, hydrogen enhanced localized plasticity and decohesion. While hydrogen-trapped sites increased the HIC susceptibility, a kind of surface corrosion occurred in the tensile tested gauge length, due to the high pressured hydrogen atmosphere. The surface corrosion was composed of HIV (hydrogen induced voids) and HIC occurred by linking of the voids. Typical SOHIC (stress oriented HIC) propagated to the inner areas by linking of planar cracks normal to the primary propagation direction. Carbides at grain and lath boundary of the X70 bainite acted as initiation sites of the voids, followed by cracking without any phase transformation. The relation of hydrogen induced defects and grain boundary carbide is discussed in relation to hydrogen corrosion and safe application in hydrogen system.